Avoid nil-pointer panic when Rego template source is missing

[sozercan]

Motivation

• GetSource in the Rego schema driver dereferenced code.Source without checking for nil, allowing a malformed ConstraintTemplate with engine: Rego and missing/null source to cause a nil-pointer panic and crash the process.

Description

• Add a nil guard in constraint/pkg/client/drivers/rego/schema.GetSource to return ErrMissingField when code.Source is nil or code.Source.Value is nil, preventing dereference of a nil pointer.
• Add unit tests in constraint/pkg/client/drivers/rego/schema/schema_test.go to cover both Source: nil and Source with an empty Anything value and assert ErrMissingField is returned.
• Preserve existing behavior for valid source payloads and error semantics for bad types or missing required rego field.

Testing

• Ran go test ./pkg/client/drivers/rego/schema from the constraint/ module and the package tests passed successfully.
• Attempting to run go test from the repository root path that is not a Go module produced a module initialization error, so tests were executed from the constraint module instead.

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Codex Task

[Copilot]

Pull request overview

This PR hardens the Rego schema driver against malformed ConstraintTemplate inputs by preventing a nil-pointer panic when the Rego template source is missing.

Changes:

• Add a nil guard in schema.GetSource to return ErrMissingField when code.Source or code.Source.Value is nil.
• Add unit tests covering both Source: nil and Source with a nil Value.

Show a summary per file

File	Description
constraint/pkg/client/drivers/rego/schema/schema.go	Adds early validation to avoid dereferencing a nil Source/Value and returns a consistent missing-field error.
constraint/pkg/client/drivers/rego/schema/schema_test.go	Adds regression tests to ensure missing/empty source returns ErrMissingField rather than panicking.

Copilot's findings

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• Files reviewed: 2/2 changed files
• Comments generated: 0

[JaydipGabani]

lgtm

[JaydipGabani]

@dependabot rebase